We report on two siblings with a severe neonatal form of spondylometaphyseal dysplasia (SMD). Similar cases have been reported in four publications. Analysis of pedigree data from the original and present families suggests an autosomal recessive mode of inheritance, although parental gonadal mosaicism is also possible. The similarities in the phenotype between our patients and spondyloepimetaphyseal dysplasia congenita (SEMDC) and spondyloepimetaphyseal dysplasia Strudwick (SEMDS) type, indicated that these patients could have a defect in the COL2A1 gene. Molecular analysis of genomic DNA of these patients excluded this gene. Another potential candidate gene PTHR1, was also analyzed in the selected regions and no mutation was found. This gene is probably causative in the Jansen type of SMD, which shares some phenotypic features with the siblings whom we documented. Our results indicate that a new candidate gene for the reported form of SMD should be sought. © 2009 Wiley-Liss, Inc.